Pressure control system for aircraft cabins



mi 17 1951 B. E. .55. AR 2,54%73 PRESSURE CONTROL SYSTEM FOR AIRCRAFTCABINS Fi May 15, 1946 6 Sheets-Sheet l 'INVENTOR. 52/65 i fiz We r IA'jj y, L

April 1'7, 1951 B. E. DEL MAR PRESSURE CONTROL SYSTEM FOR AIRCRAFTCABINS 6 Sheets-Sheet 2 Filed May 15, 1946 INVENTOR. ave: 5 if; 4&2,

/ ve/ /Y April 17, 1951 B. E. DEL MAR PRESSURE CONTROL SYSTEM FORAIRCRAFT CABINS 6 Sheets-Sheet 3 Filed May 13, 1946 G 5 7 3 9 Z w M41 fzr w V a Z 9 61 m? 3 m w m a 9 9 7 5 fifl is z 9 4 2 M 3v 7 m 4 6 7 e IH m m m mm r, u z 0 o o m 7/ fl 3 Q. H m /v.

u u n M n m I s e v." m u M r m INVENTOR. ears 5 D54 We BY April 17,1951 B. E. DEL MAR j PRESSURE CONTROL SYSTEM FOR AIRCRAFT CABINS FiledMay 15, 1946 G-Sheets-Sheet 4 -.Q Wig MAXIMUMS TYPICAL CONTROLLED BYPRESSURE LIMITS REGJLATOR I8 D3796. 4;L\ I PRESSURE 4 276 IN.HG.|6IABSOLUTE CURVE DATA FOR TYPICAL CABIN PRESSURE SCHEDULES CONTROLLED BYRATIO-TO- 1 FLIGHT REGULATOR. NET RAT a CONTROL PRESSURIZING RATIO LIMITcABlN /275 -CURVE ALT. SETTlNG FLIGHT All SETTING FL; T PRES. CHANGE A2000 FT. nsoo FT. .0

6 B H I5 500 c H 25 200 50 0 e000 r1300 0 F u 28,5 irlrltlli rl 2 4 s 8l0 l2 l4 I6 20 22 24 26 2a 30 52 3436 3a FLlC-HT PRESSURE ALTITUDETHOUS. FT.

INVENTOR.

5 0 M -1 A Bf? April 17, 1951 B. E. DEL MAR PRESSURE CONTROL SYSTEM FORAIRCRAFT CABINS 6 Sheets-Sheet 5 Filed May 15, 1946 M 4 27 7 ow 7 2m W MMg Aw; 5 l 2 6 9 a d, 06 o M W/Q Z 2 a .0 M 9 M M 3 x 4 5 C/ x 4 9 1/ nw 5 a a a 7 M H I 7 INVENTOR. 52w; 5 P-2 M414? fOIPA/EY April 17, 1951B. E. DEL MAR PRESSURE CONTRQL SYSTEM FOR AIRCRAFT CABINS 6 Sheets-Sheet6 Filed May 15, 1946 I24 I32 200 208 2l6 TIME FROM START OFFLIGHT-MINUTES Patented Apr. 17, 1951 Bruce E; Del Mar,iLos.Angeles;.Calif;,assignor;to

DouglasiAircraft Gompanm lnc Santa; Monica,

Calif.

Application May 13, 1946-; Serial'No; 669,-366" 30 Claims. 1

This invention relates to the controlofpressure in sealed cabins such-asthoseof aircraft in order to isolate flightpersonnel from uncomfortablechanges in pressure and low values of absolute pressure experienced athigh altitudes.

It is well known that pressure changes encountered in flight maycauseconsiderable discomfort; and that the low pressures encountered athigher altitudes necessarily limit the flight altitudes'of aircraft notprovided'w-ith supercharged cabins. Considerabledevelopment has beenundertaken in the past toward providing means to control thepressur inaircraft cabins along certain predetermined schedules.

It was soon found that it Was not practicaltol maintain the absolutepressure in the cabin at values normally encountered at airportlevelsdue to extreme penalties in weight and power. Furthermore, it wasfound that passenger sv could readily tolerate the lowerabsolute'pressurecorresponding to an intermediate altitude somewhatabove airport levels. Initial efforts were directed toward simpl ymaintainingv cabin pressure constant at an intermediate value wheneverflight above that prescribed level 'was attempted and in conjunctiontherewith, a schedule of maxi.- mum pressure difference between thecabin. and

the: flight atmosphere was prescribed at a cone,

venient value so that/whenever flight was attempted. above apredetermined level, the absolute pressure in the cabin thereafter wouldascent'and descent of the -aircraftibut also to" reset. these-' controlsunder continual observation during ascents and de'scents: in order toattempt to isolate the cabin from uncomfortable pressure influencescaused by change of flight pressure altitudes when approachin limitdifferential pressure or uponapproach to the landingfield; Failure onthe-part -of-' the operator toaccurately predict the ascent ojrdescentpattern of the aircraft-inflight with this type of-' controlimposesunreasonable discomfort on the passengersby greater rates-ofpressure change than necessary. The constant" attention requiredifreasonableresults are to beexpected constitutes a costly andecrease andmaintain. the pressure difference 1 results in this direction the nextstep taken was to add to the aforementioned control a time rate of cabinpressure change control whereby cabin pressurecould be set to increaseor decrease in increments corresponding to a chosen rate of altitudechange until the intermediate pressure value was attained on ascent, or'until cabin pressure was equalized with flight pressure during descent.This mode of. control has proven entirely inadequate as the operatormust continually fly the cabin pressures by setting its controls notonly at critical times upon starting noyance and much more shouldreasonably be expected from I a truly automatic control.

Systems have also been previously proposed which controlled cabinpressurein some-predetermined relation to the change in pressure ofthe-flight atmosphere so that the unreasonable burdenof resettingthecabin pressure controls at critical times could be avoided. Suchsystems have failed to vary cabin absolute pressure for the occupantsin-terms of the veryfunction upon which all ascents or descents are ofnecessity guided: an'equal increment of flight altitude" per unit oftime. Instead such systems have pro-- posed to vary cabin absolutepressure in proportion to changesin flight pressure and since airbecomes rapidly more dense at low altitudes, the cabinpressure duringascents and= descents change very rapidly at the low altitudes andconsiderably less rapid during ascents or descents at the higheraltitudes.

The control system: of the present invention obviatesthe diflicultieshad with previously proposed devices by providing: means for controllingthe absolute pressure within the cabin as a straight line function ofthe altitude of the aircraft; that is, altitude used inthestandard-aeronautical sense as meanin altitude based on absolutepressure in the standard international atmosphere and altitude as usedherein is intended to mean pressure altitude.

The cabinpressure control efiected by the-control device of the presentinvention automaticallyproduces without contingency or guesswork theslowest and therefore the most comfortable pressure change rate for thecabin occupants during descents and ascents generally encountered-inaircraft flight operations. This is true becausethe factor controllingthechangeof cabin absolute pressure during either ascent ordescent,oftheaircraftis-the change of altitude of the aircraft. Sincecommercial aircraft have 3 so-called placard or limit speeds, descentscannot be made faster than the rate which will produce the limit speedand it is conventional airline practice to descend in accordance withequal increments of altitude per unit of time. Similarly on the ascentthe relatively constant availmeans for limiting either the cabindifferential pressure or the ratio between cabin and flight able powerfrom modern supercharged engines within the normal flight range makes itreasonable to use the excess power over that needed in overcoming levelflight gravitational lift and drag forces to increase the aircraftaltitude in equal increments per unit of time. Thus the typicalscheduled airline climb is one of constant flight speed and constantrate of altitude increase. And now since passenger pressure comfort inthe cabin may be measured in terms of pressure change rates, it isclearly most desirable to control the absolute pressure within the cabinas a straight line function of the pressure altitude of the aircraft.

This invention also provides in a cabin pressure control system meansfor maintaining smooth automatic regulation of cabin pressure regardlessof flow surges inadvertently imposed on the cabin ventilation system.These means actually anticipate the surges in the air delivered to thecabin and provide the necessary sensitivity to counteract and fullyeliminate the adverse ef: fects which would otherwise be highlydetrimental to passenger comfort. Furthermore, cabin pressure schedulechanges initiated by resetting the controls if and when desired may alsoimpose flow variations at the discharge valve of the cabin. It is highlyimportant that these surges be smoothly counteracted before they affectcabin comfort, and that follow-up action to the pressure controls beprovided to prevent hunting. This invention includes means to stabilizethe control and give follow-up action with the special feature of notchanging the normal cabin pressure control value in so doing.

To obviate the necessity of the operator of the pressure controls beingforced to refer to special tables or charts, the present inventionprovides a control-setting apparatus on the cabin pressure controlsystem which at all times will furnish to the operator a simple visualpicture of the simulated cabin altitude which may be expected at anyflight altitude during progress of the schedule and also shows thelowest cabin altitude which can be maintained within capability limitsof the structure and supercharger apparatus at any given flight altitudefor the particular aircraft. This control setting apparatus alsoconstitutes a means of setting the control schedule for any flight longbefore the flight is started and to set the ratio between changes inabsolute cabin pressure to changes in pressure altitude of the aircraftto the slowest cabin pressure change rate feasible during any givenpressure altitude change.

The control system of the present invention also includes means forautomatically controlling the time rate of pressure change for thecabin. This means, although ordinarily automatically operable, can be soset as to permit full manual control to be assumed in emergencies orduring unusual flight procedures as may, for example, be undertakenduring test flights or weather disturbances. The time rate of pressurechange control means is normally operable in the preferred embodiment ofthis invention at predetermined limits to supervise and veto any actioninstigated by normal operation or resetting of the ratio-toflightcontrol means or the control action of the pressures which may tend tomake the cabin pressure change rate exceed the predetermined limits.Although the rate of pressure change means is normally an overridingcontrol it may be set to assume primary charge of the rate of cabinpressure change when desired.

The control system of the present invention also includes a controlmeans which will limit the maximum cabin diiferential pressure to valuesdetermined by the safety limits of the structure of the particularaircraft in which the control is incorporated even though theratio-to-flight or time rate of pressure change control means might tendto exceed these limits during flight operations at altitudes abovelevels initially expected.

To prevent overloading of the supercharger equipment or other airdelivery means employed When flights are attempted at altitudes abovethose for which normal operation is intended, control means are alsoincorporated which will assume primary control of pressure within thecabin if a predetermined ratio of absolute pressure between the cabinand flight atmosphere tends to be exceeded and this control will limitthe absolute pressure ratio to the predetermined maximum value.

Another object of the invention is to provide means to automaticallydepressurize the cabin at a comfortable pressure change rate in the caseof an emergency landing at a field above that originally intended or incase of inadvertent or even intentional settings of the ratio-to-flightcontrol means at pressure altitudes below the pressure altitude of thelanding field. These means are also operable to obviate thepressurization of the cabin by such inadvertent settings while theaircraft is parked and so long as the aircraft is supported by thelanding gear.

To prevent temperature of the air within the cabin from exceeding somepredetermined temperature due to heating of the air by compression,temperature responsive means are incorporated in the system forautomatically decreasing the pressure within the cabin when thetemperature therein reaches the predetermined temperature. This thermoresponsive means is subject to the time-rate-of-pressure-change controlmeans in a similar manner to the means controlled by the landing gearthus preventing the pressure in the cabin from changing at a rate inexcess of the rate imposed by the time-rate-ofpressure-change controlmeans.

Other features and advantages of the present invention will be apparentfrom the following description taken in connection with the accompanyingdrawings, in which:

Figure 1 is a diagrammatic View showing the cabin pressure controlsystem as applied to a typical aircraft cabin;

Figure 2 is a perspective view of the preferred embodiment of myratio-to-fiight pressure regulator with a portion of case broken away tomore fully illustrate the same;

Figure 3 is a perspective view of the most important operating parts ofmy ratio to flight cabin pressure regulator shown in Figure 2;

Figure 4 is a side view partly in section of the regulator shown inFigure 2 Figure 5 is a front View showing the dial face and hands of theregulator shown in Figure 2;

Figure 6 is a graphic plot of the pertinent pressure control relationsbetween cabin pressure and a charged;

anaeaa flight pressure; the l'atter-being represented as flightaltitude;

Figure-Pisa View of the di'al' and front face of my; time rate ofchangecabinpressureregulator shown schematically in the system of Figure1; Figure 8 'isa sectional view partly inelevation of nry time rateofchange cabin' pressure regreferring now. to the drawing andparticularly Figure 1 thereof; is shownascontrolling the pressure withina sealed aircraft cabin 2|. Air" is directed. into the. cabinthrough anair duct 22-arranged to delivera flow of ventilation air tothe cabin"from a superchargingblower 23. The blower 2.3 is arranged to besuppliedwith air' from aramduct2an'd-is' driven through a-shaft 25 by a speedcontrolled prime mover- 26 in such manner. that a substantially constantrate 'of ventilation. air. flow is supplied through the duct 22 to thecabin2l'.

Although one cabin. supercharging blower is shown for simplifying theillustration of the now preferred'embodiment of 'theinvention, itis'obviousthatdual blowersoperating in parallel could be used; 7

A' check valve. 21 is mounted within duct; 22 andis so fformedthat' itisnormally opened' by the flow of air through theduct, but which willclose and seal the duct to maintain cabinv pres sure in the event'offailure ofair flow in. the duct 22.

If desired, some conventional temperature regulating means may bemounted within the duct to controlf'the' temperature of theincorning airto maintain the air supplied'to the cabin ata comfortable temperature.The air temperature conditioner 28"may also include its own automatictemperature. control means operated in response to thermostatsorsimil'ar controls disposed'within thecab'in',

A'discharge duct 29 leading to an outlet 31 in the cabin, wall isprovided for the discharge of air from the cabin. The outlet 3| ispreferably locatedlon thecabin wall in a region where the pressure alongthe wall is .due to surface air ve-' locities slightly. less thanthat'ofthe aQmbientatmosphere. Interposed in or as a part of the duct;29.is a.valve 32which in its variousoperating positions varies the outlet areaandthus provides. any desired throttling, of cabin air dis- Sinceasubstantially constant rate of air flow enters the cabin through theinlet duct 22, cabin pressure will be increased when the discharge valve32 is closed or moved toward the closed position so that the. airdischarge is less than the air flowing into the cabin. Onthe other hand,if the valve is opened so that the air discharged is greater. than theflowof incoming air, the.pres. sure Within the cabin will be decreased.

Although any means desired may be used to control the valve 32, in theillustrated embodiment of the invention the control of this valve iseifected through an operating; linkage 33'.- a worm gear 34 carried by ashaft 35, and a prime mover 361 which in the embodiment; illustrated isshown as an electric motor of the reversible split fieldseries type.Although-fort the purpose of illustrating the invention, the prime mover36 has been shown as an electric motor; obviously hydraulic orpneumatio'power could be-substi tuted without departing'fromthe scope of thisinvention. valve drive in' order to substantially relate: the number ofmotor-turns" to change a'g'iven increment of cabin pressure: overthe-full range of valve positions; The motor 36n 1ay' berenergizedeither'througli a field coil circuit 3l" or: a field coil circuit 38''by power from somesuitablesource such as the battery39' to open or closethevalve 32, depending upon" the direction of'rotation'of the motor.

A manually operable master switch 4| is -interposed bet-ween themotor 36and the battery 39 andcon'trols the motor circuit. Directional controlof the motor, and consequently the valve 32 may be effectedby selectedoperation of a controlswitch 42; In the embodiment of the" inventionillustrated, movement of the switch arm into engagement with theleft-hand contact as viewed inFigure-l closes the valve and-engagement"of the. arm with" the right hand contact opens'the'valve.

To prevent over-travel. of the valvej limit switches and 44' areconnected-into the field circuits of the motor 36. The" limit" switch 43is. adapted to open the field coil circuit'38when the valve 32" reachesits fully open position, while the, switch. will open the field coilcircuit 31 when the valve. reaches its. fully closed position.

, Some direct. manual. control. for thevalve 32, although not. shown,may. be added or substi tuted' for the. master control switch if sodesired. 1

' With. the directional control switch 42. in its neutral. position, as.shown inFigure. 1, automaticcontrol of cabin pressure isv effectedthrough a-control relay. 45. Relay 45 as shown for illustrative purposesisessentially a power amplifier inwhichv very small cur-rents from. abattery 46- through two-relay field coils ll andfiiB can be activated bythe control circuits 49. and 5 respectively, tobring about a flow of.relativelylargecurrents inthe motor field circuits 3'1 and 38respectively. Energization of the control circuit lll and'itsassociated'coil 4.! willcause the. armature 5i]- of the relay 45 to.move into engagement-witha contact 52in the field coil circuit 31" toenergizethe same. Energization of the field coil 37 causes the motor 36to close the discharge valve- 32 and similarly activation of the controlcircuit 51- will cause the armature 50 to move into engagement'withthecontact 53 and result in opening movement of the discharge valve 32;.

. The. relay 45 is: provided with twocentering springs '54 and. 55 whichnot only hold the armature '50 inthe: open'position shown in whichneither controlcircuit is activated, but also moves thearmature into itsneutralposition whenever both control circuits are energized. Although asingle spring loaded relay'element is shown, dual relay elements inseries may be substituted" if desired in the amplifier system to producethis circuit-cancelling action. Amplification of the discharge valvepower by meansv of variable lowrange resistance, variable capacitance orvariable inductance may also be substituted for the grounding type ofcontrol amplifier illustrated The linkage 33 1s included in the withoutdeparting from the scope of the invention.

It should be understood now that activation or grounding of the controlcircuit 49 will result in the discharge valve 32 moving toward itsclosed position to increase cabin pressure, and that grounding of thecontrol circuit will result in opening movement of the valve 32 to bringabout a decrease in cabin pressure. Control circuit 49 may be activatedby the action of either a ratio to flight cabin pressure regulator 56 ora time rate of change cabin pressure regulator 51. The control action ofboth regulators 56 and 5'! is subject, however, as will be hereinaftermore fully explained, to the overriding action of another primaryregulator, a cabin pressure limits regulator indicated at 58 in Figure1.

The control circuit 5| may be activated to call for decreased cabinpressure by any one of three primary regulators 56, 51 or 58, as well asa cabin overheating thermostat 59 or a landing gear switch 69. Specifictransaction of each of these regulators and control elements will bediscussed in turn.

The ratio to flight cabin pressure regulator 56 normally the most activeof the primary regulators comprises, referring now to Figures 2 and 4, asealed case 6| carrying a frame 62 on which is mounted an assembly ofaneroid capsules 63 and an assembly of differential capsules 64. Acontrol arm 65 is so pivotally mounted within the regulator that it ismoved by the expansion or contraction of either or both the capsuleassemblies 63 and 64. The one end of the arm 65 is interposed between apressure increase control contact 66 and a pressure decrease controlcontact 61. The contact 66 is insulated from the case 6| and isconnected to control circuit 49 by a lead 68. The contact 61 is alsoinsulated from the case BI and is connected to control circuit 51 by alead 69. The control arm 65 is electrically grounded by means of a leadIII. The leads 68, 69 and 19 are connected to a suitable terminal socketII carried by the frame 62 l and adapted to receive a conventionalattachment cap when the instrument is mounted in the aircraft.

The control arm 65 is balanced between and separated from the contacts66 and 61 by the cap sule assemblies 63 and 64 whenever the pressurewithin the cabin corresponds to the control schedule of the regulator56. If cabin pressure, admitted through aperture I2, is substantiallygreater or less than scheduled, control arm 65 engages pressure decreasecontact 61 or pressure increase contact 66 respectively, and controlvalve 32 is moved to a new position to bring about the predeterminedscheduled cabin pressure.

The aneorid assembly 63 is of the conventional jacketed and sealedelement type, and is'rigidly mounted at its base to a slide I3 which inturn is adjustably mounted on one leg of an intermediate slide member14. Adjustment of slide I3 on the intermediate slide I4 may beaccomplished by means of a slide clamp 75 and an adjusting screw I6. Theaneroid assembly 63 is flexibly supported on its expandable end by aresilient support 'I'I attached to the slide 13. The intermediate slide14 is slidably mounted on the frame 62 and is held in abutment therewithby a slide bar assembly l8. Adjustment of the position of theintermediate slide 14 on frame 62 may be made by a lead screw I9. Leadscrew I9, threaded into the intermediate slide at one end as shown issupported and held in position coaxially by lock nuts 86 straddling aframe pillar 8I. The lead screw carries a gear 82 at one end thereof andwill be rotated when the gear 82 is rotated. The gear 82 engages a gear83 on a dial setting shaft 84 also supported on the frame 82. The shaft84 pierces the regulator case BI through an air-tight seal and carriesat the outer end thereof an adjusting knob 85.

Within the case 6I and also mounted on the shaft 84 is a drive gear 86which drives through an idler gear a pinion 88, carried by the inner endof a short hollow shaft 89 journaled on the frame 62. Fixed to the outerend of the shaft 89 by any means desired is a dial 90 carrying asuitable scale. The position of the dial 90 with respect to any adjustedposition of the intermediate slide I4 may be made by adjustment of theposition of the lock nuts 86. It is therefore possible to set the zeroposition of dial 90 without interfering in any way with the desiredadjustment of contacts 66 or 61.

The differential pressure capsule assembly 64 is of the conventionalthin Walled corrugated disc type and is rigidly mounted at its base to aplate 9| and an adjustable support ring 92. The support ring 92 is inturn mounted on a capsule slide 93. The position of differential capsuleassembly 64 with respect to the slide 93 is made adjustable by screws 94and 95 carried by the slide 93. The position of the slide 93 withrespect to frame 62 can be varied by means of a sliding clamp 96 andadjustment screw 91. The capsule assembly 64 is flexibly supported onits expandable end by a spring support 98 attached to an integral fingerof the base plate 9|. A tube 99 leads from a fitting I carried within anaperture formed in the rear of the case 6! to the interior of thecapsule assembly 64. The fitting receives the one end of a tube IOIwhich leads to the exterior of the aircraft so that the interior of thecapsule assembly 64 is in communication with the flight atmosphere.

The control arm 65 comprises an elongate metallic member arranged toswing one end between the contacts 66 and 6? about either of the twoindependent but parallel hinge axes. The distance between these hingeaxes is made adjustable by a spreader screw I82. Static balance of thecontrol arm is made possible by an adjustable counterweight I03. Theprimary support for the control arm 65 is provided at one of the hingeaxes by jeweled bearings I64 carried by the free end of an arm of anL-shaped support I05.

I The support I is hinged at the opposite end by a pin I66 mounted inlugs IIJI formed integral with the frame 62. The other hinge axis of thecontrol arm 65, as best seen in Figures 3 and 4, is established by a pinI68 carried by one end of a link I89, the opposite end of which ispivotally connected by a pin III to a lug II2 carried by the expandableend of the aneroid assembly 63.

From the structure thus far described it will be noted that an expansionof the aneroid assembly 63 would tend to rotate the control arm 65toward the contact 66 while a contraction of the aneorid assembly tendsto cause rotation or movement toward contact 61. If L-shaped support I05is held against movement, control as infiuenced by the contacts isaccordingly such as to maintain a constant value of pressure within theregulator case.

Associated with control arm support I05 there is provided a means forobviating any action on control arm 65 because of expansion orcontraction-ofthe ;differential pressurecapsule assembly 64, as well asfor-selecting any reasonable degree of action therefrom to combine withaction from the capsule assembly 63.

' The components of :the nowprefe'rred I embodiment of this. meanscomprises, as best seen :in Figure '3, Ya ratio control link ;I I3;pivotally:attached .at 'one extremity by a. pin II 4 'to theuL- shapedsupport I05 and at the other 'end topan elongate :pin H5. The.OIJPOSitCf-BIldS'Of the. ;pin II5 are pivotally mounted in the armsofaa1yoke I16. A link III .pivotally connected at one'end by agpin II8toa lug I.I9 carried 'by'ithe capsule assembly 64 is formed atth'eopposite end with a'bifurcated member I26. :which' stradles the end ofthe link II'3, thelegs rof the bifurcated memher being formed withalign-ed :openings 'for rotatably passing the pin II5.

The free ends of the'arms of the yoke 116 .are connected by suitablepins I2l1to thetfree ends of the arm of a U-shapedbracket I-22,::whichin turn are connected adjacent the bow :of'the :U to pivot "pins I23carriedibythespillars 8! .of the frame'62. The positionfof the U-shapedbracket may thus be'adjusted about the .axis siof the ilpins I23, andthis adjustment ispreferably accomplished by-means of -aaworm gear I 24carried by a shaft I25 and 'a sector gear' I26 car-ried by the bracketI22 and mountedbetweenthe same and one'of the pillars 8I. I

Fixed to the shaft 'I25,=referring now to Figure 4, is a gear IZI'whichmeshes with'asecondgear I28 carried byash'aft I29 supported on the frame62. The-shaft I29 pierces-"the regulator case -6'I throughan integralseal and'carries at the outer end'thereof an adjustingknob I31. AgearI32 mounted on the shaft I29 engages an idler' gear I33 meshing with'apinion I-34 carried by a stub shaft I35. This shaft iscoaxiallymountedtrelative to the shaft '89 and -'carries at*the outerend thereof a circular disc "I36' havingintegrally formed therewith apair of hands 'I3'I='and I38. As the hands I31 *and I38 areintegrallyformed with the-disc I36, the anglebetween them'is fixed andcanbe predetermined-v 'The'coopera'tion between the hands I31 and I38and the dial flfl will be hereinafter more 'fully expla'ined.

It should be noted '-now' that "all of the p'ivotal axes of thecomponent elements of the regulator 56 as d'efined'by bearings "I 04,pins I66, I08, II4, III, I I5, IIB- and -I2-3 are substantiallyparallel. The distance between centers of pins H4 and 115 is madeequivalent in length to the diiferencebetween centers .of pins I Z'I'a-nd I23. "Furthermore, the-axes of pins 'I23:are:coincident with theaxis of the-elongate pin I I5 whenever the differential -pressurecapsule assembly' 64 subjected to zero pressure difference between theinterior and exterior of the cabin. Since the separation between theaxesdefined by the pins I14 and II 5 is equivalent toithaties'tablishedby pins I2I "and I 23, an-adjusted position :of the U shaped bracket I22 may be'cho'sen' for which -:the hinge .center of axis of pins I2I iscoincident with "the axis of pin' II' I. When the bracket I22is'sopositioned the pathof :movement of pin" I'I5 created by a contraction of:thecapsule assembly 54 is concentric with :pin I I4 and controlarm-support I65 is substantially (locked against movement regardless ofthe differential'gpressure and control arm 65 'is'then subject only toabsolute pressure.

?Now, .if the bracket-I22 is; moved to a position :as shown in Figure 12-or =4, as soon as :any .con-

traction -:of 33 1 855111 6 --capsule :64 @occurs 'such "as would -occurif i the pressure in flight pressure tubew99uwere-rallowed to becomeless than the pressure'in the case 6I; thenpin -I-I5'tends torotatearound the new position of pin I2I and the link I I3'will;pul1-thesupport I05 toward the differential pressure'capsule assembly, thuscausing control arm'65 to-rotate about gpin I68 toward engagement withcontact 61. The "farther the control arm support bracket I22 is "movedfrom the position shown in Figure :3 the greater the action of thedifferential pressure capsule assembly on the control arm. This functionof the adjustment of the bracket I22 will be'better understood afteran'overall description of the regulatory action of the system has beenmade.

The time rate of change cabin pressure regulator 5'1, referring now toFigure I8, comprises a sealed :case 442 :inwhich is mounted a support--ing frame I43. A slide I44 mounted on the frame I43 is movablethereonfor :adjustably position-:- ing a differential pressure capsuleI45. The capsule I45 is attached to the slide I44 'byrmeans of anadjustable support ring I46'and a base spool I41. An adjustment screwI'4Bpermitsa'de justment of the slide I 44 and the capsule with respectto the frame I43. An adjustment screw I49 permits the capsule to bevertically adjusted, as viewed in Figure 8.

The interior of thespool I41 communicates directly with the interior ofthe capsule I45 and through tube I5I toa fitting I52 mounted in :anaperture I53 formed'in'the'case I42.

A control arm I54 carrying at one-end thereof a contact I55 is pivotallymounted adjacent the opposite end thereofto =a finger I56 integrallyformed with the frame I43. To counterbalance the arm I54 a weight 151having a tapped aperture therethrough is threadedly mounted to the oneend of thearm. A link I58 pivotally interconnects control-arm I 54 andthe expandable end of the capsule I45, the link being pivotallyconnected at-one-endto the arm and attheoppo'site end toa'lug I59carried by the capsule.

The control arm is electrically connected by a lead I 60a to a circuitI60, the purpose of which will be hereinafter more fully explained. The

control arm 'is so mounted within the 'instru- I ment that the contactI55 is movable-between a pressure increase-contact I6! connectedtocontrol circuit 49 by-a lead49a anda pressure decrease contact I62connected to control circuit 5| by a lead 51a. The contact I 61 ismounted on an arm I63 formed with athreaded aperture receiving athreaded shaft I64 journaled'in bearings I65 carried by the frame I43.Rotation of theshaft I64 and adjustment of the arm I 63 and contact I6Iis accomplished through a shaft I66 'rotatably supported on the frame,and carrying at the inner end a gear I61 of a bevel gear set, the othergear I68 of which is carried by the upper end of the shaft I64.

The shaft I66 carries at the outer end thereof a knurled knob I69 topermit the shaft I66 to be rotatedto bring about the desired adjustmentof the contact I6I.

The contact I62 is carried by an arm I'II similar'to the arm I63 andformed with a threaded aperture for receiving a rotatably mountedthreaded shaft I'I2 journaled in bearings I13 carried by the frame ofthe instrument. The lower end of the shaftI'IZ carries a bevel gear*I'I4engaging and meshing with a gear I15 carried by :a shaft I 76 rotatablymounted on the frame of the instrument. Thelshaft I'I6also carries at 11shaft I16 to be easily rotated to bring about any selected adjustment ofthe contact I82.

The shaft I66 carries. a gear I18 which, through an idler gear I19,drives a pinion H. The pinion I8I is carried by a hollow shaft I82 whichpasses through a dial I83 carried by the frame of the instrument andsupports at the outer end thereof a hand or needle I84 which serves asan index means for the scale of the dial I83.

It should now be seen that manipulation of the knob I89 adjusts theposition of the contact IBI and simultaneously sets the hand I84 toindicate the climb limit set by the adjusted position of the contactI6I.

The shaft I18 carries a gear I85 which engages and drives anintermediate gear I89 which in turn meshes with and drives a pinion I81carried by the inner end of a stub shaft I88 coaxially extending throughthe hollow shaft. The shaft I88 carries at the outer end thereof a handor needle I89 movable over the dial I83. As will be understood, rotationof the shaft I16 through manipulation of the knob I11 simultaneouslyadjusts the position of the hand I89 and the contact I62 to provide adescent limit setting.

Insulated stops I9I are provided on inturned fingers of the arms I83 andHI to limit movement of the contacts I6I and I82 toward each other andin the now preferred embodiment of the invention the two contacts can bebrought together to a position in which they are separated a distanceslightly greater than the thickness of the contact I55 carried by thecontrol arm I54.

A capillary tube I93 leads from the interior of the case I42 to theinterior of the hollow spool I41 which is in direct communication withthe interior of the differential pressure capsule I45. Whenever pressurein the tube I93, spool I41, and pressure capsule I45 changes rapidly, apressure difference is built up across the capsule I45 due to therestriction to air flow in the small passage of the capillary tube I93,and the control arm I54 is subjected to large angular deflection.Similarly, very slow changes in pressure diiferential across the capsuledevelops small angular deflections of the control arm I54.

In the illustrated embodiment of the present invention counterclockwiserotation of the control arm I54, as viewed in Figure 8, is brought aboutby decrease in the pressure of the air within the tube II and clockwiserotation of the arm is brought about by a pressure increase within thetube I5I. In accordance with conventional practice in aircraft rate ofclimb instruments, the capillary tube I93 may be of or filled with amaterial which increases its restriction to air flow in accordance withreduced air densities so that whenever so desired compensation may beobtained to permit relatively larger deflections of the control arm I54for a given pressure difference at high altitudes. When so compensated agiven control arm deflection corresponds directly to a constant value ofaltitude change. Furthermore, temperature compensation may be providedas in present day aircraft rate of climb instruments by supplying anauxiliary gas filled diaphragm or a bi-metallic strip adjacent thepressure capsule I45.

The zero position of contact I9I is such that a very slight clearance ishad between the same and the contact I55 when the control arm I54 is inzero pressure change position. A contact I94, carried at the free end ofa spring arm I95 attached to a finger I99 integrally formed with theframe I43, is so held by the arm I relative to the contact IBI that forall positions of the latter from just beyond its zero adjustmentposition into the climb rate of control arm I54, engagement ismaintained between contacts I6I and I94 by the resiliency of the springarm I95. The contact I94 is electrically connected by a lead 68a to thecircuit 99 through the resilient arm I95 and is thus electricallyconnected to control circuit 49.

However, upon adjustment of contact I6I to its zero position and intothe descent range by proper manipulation of the knob I69 the engagementbetween contacts I6I and I94 is broken by a screw I91 adjustably mountedon a lug I98 integrally formed with the finger I99 and extendingsubstantially normal thereto.

The zero position of contact IE2 is such that a very slight clearance ishad between the contact I55 when the control arm I54 is in the zeropressure change position. A contact I99, connected by leads 69a and 99to circuit 5!, is carried at the free end of a resilient finger 29!having the opposite end anchored to an arm 292 integrally formed withthe frame I43 and projecting upwardly therefrom as viewed in Figure 8.The resilient finger 29! is so positioned that for all positions ofcontact I62 from just beyond its zero adjustment position into thedescent range of control arm I54 engagement is maintained betweencontacts, I82 and I99. Upon adjustment of the contact I92 to its zeroposition or anywhere into the climb range this engagement is broken bythe screw 293 adjustably mounted in a lug 294 extending substantiallynormal from the finger 292.

The cabin pressure limits regulator 58, referring now to Figure 10,comprises a sealed case 299 of insulating material in which is formed atapped aperture for receiving one end of a tube or conduit 299a leadingto the tube I9I. The case houses a suitable frame 295 on which ismounted a hollow spool 296. A differential pressure capsule assembly 291is rigidly secured to the one end of the spool 299 and the interior ofthe latter directly communicates with the interior of the capsuleassembly. A tube 298 leads from a fitting 299 in communication withconduit 29911 to the interior of the spool 295 to the end that theinterior of the capsule is subjected to flight pressure. A control arm 2I 9 is pivotally mounted adjacent one end thereof to an L- shapedsupport 2II suitably supported by the frame 295. To counterbalance thearm 2I9 a weight 2I2 is threadedly mounted to the one end of the arm formovement longitudinally of the same.

The control arm 2I9 and the capsule assembly 291 are pivotallyinterconnected through a short link 2 IS the opposite ends of which areconnected respectively to the capsule assembly and the control arm 2I9through suitable Pivot pins.

The position of control arm 2I9 is thus varied by expansion orcontraction of the capsule assembly 291. The free end of the arm U9 is,thus movable between a pair of contacts 2I4 and 2I5 carried by rods 2I6and 2I1 respectively, adjustably carried by fingers 2I8 and 2I9respectively, supported on the frame 295, and insulated therefrom. Abumper 229 formed by an extension of finger 2I9 limits the angulardeflection of the control arm 2I9. The position of the contacts 2| 4 and2I5 can be selectively adjusted by rotation of the rods 2I6 and 2Hthrough engagepressure "capsule assembly 201. ratio function' is derivedfrom the action "of 'an 'ment of "the tool receiving members carried atthe 'oneend of each'of therods.

The leads"22l,222, and "223 respectively, electrically connected tocontacts' 2l5 and 2M and the control arm 2l0, respectively are attachedto 'a suitable lead-in'receptacl-e 224 which takes an attachment cap(not shown) carrying contacts connected to circuits -I60,-5| and asuitable grounding connection respectively.

Control arm 2!!) is held in engagement with ground circuit contact -2l5whenever the pressure differential acting on diiferen'tial pressurecapsule assembly is less "than apredetermined -value. If'the actingpressure differential "is greater than this limit value, control "arm 2|ll first disengages from -contact2l5 opening the control circuits ofregulators and '5'! and 'upon "further increase in "differentialpressure moves into engagement with contact .2 M which results inenergization of control'c'ircuitii "and 1 therefore opening movement ofthe control valve 32 until the limit cabin pressure is no .longerexceeded.

Cabin "pressure limits regulator '58is arranged furthermore to decreasethellimiting differential pressure in accordance with a fixedlimitingratio of the absolute pressures existing across the This limitin aneroid225 on difierential pressure capsule 201. The aneroijd capsule 2251issupported 'by'a'ring 22! carried bya'slide 228 mountedon an en- "larged.portion of the iframei205. Anadiustrnent .screw 229 Carried by an.upstanding .boss formed integral with the enlarged portion of theframe205 permits the slide 228 and the aneroid 225 to be adjustedlongitudinally of the .frame 205. A secondadjustment screw 23| permitsaxialadjustment of .thevfixed side of the aneroid 225 A-Lpost'232carwith respect [to the slide 228. ried by the expandable side of theaneroid capsule 225 .is adapted to engage against the underside of thecontrol arm 21.0 .to augment theflaction of capsule .201 .on the.control arm when- 7 ever aneroid 225 expands more than .a predelectedratio of the absolute cabin pressure to absolute .flig'ht pressure, thatis aa preselected cabin compression ratio whenever regulators 56 and .51signal for control of the valve 32, which would result in a cabincompression vratio :in excess of the value preselected. .It .will -beseen that as long .as control arm .210 is disposed between contacts .2IA .and .215, .the cabin compression ratio will remain constant,butshould the preselected compression ratio be exceeded, the action ofcapsules 2.01 :and 225 will .be .sufiicient to move the control armllliagainstcontact 2L4, resulting .in energization of control circuit,5! and opening movement of the valve to reduce absolute pressure withinthe cabin and consequently .the cabin. compression ratio.

Two associated control elements, namely, the cabin overheat thermostat5,9 and the landing gear switchBfl are include'din the cabinprcssurecontrol system as shown Jinffjigure 1. Each of these elements isconnected "to a special valve opening control circuit'23'3,'whichwhengrounde'd energizes a ratio regulator cut-out :relay 234'. The relay234 comprises, referring now toFigure 1, a field coil 235, an armature236, a spring 23?, and a pair of contacts 238 and 239. Contact 238 iselectrically connected to ratio regulator ground lead H while contact239 is connected to the control circuit 5|. The armature carries acontact which is connected to the intermediate groundlead I60. Thespring23i normally maintains the contact carried by the armature inengagement with the contact 238 so that the regulator 55 is normallyoperative. However, when control-circuit 233 is grounded as by theoverheat thermostat 59 or the landing gear switch '50, the coil 235 ofthe relay 234 is energized and the contact of the armature 235 is movedinto. engagementbetwecn the contact carried by the armature 236 and thecontact 23il. The disengagement of these contacts breaks the groundingcircuit for regulator 56 and energizes the valve opening control circuitM, which, through the valve mechanism hereinbefore described, causes thevalve to open, thus bringing about a reductionin cabin pressure or adecrease in the absolute value of pressure in the cabin. As may be notedfrom the circuit diagram of Figure 1, however, this valve opening actionis still supervised'by regulator "5'! to limit the rate of pressuredecrease in the cabin.

The overheat thermostat 55 mounted within the cabin and electricallygrounded as shown in Figure 1 comprises a bi-metallic strip 2 H carryinga contact 242 engageable with a-contact 243. "The strip is so formedthat it will move contact 242 toward contact 2E3 upon increase of cabintemperature. Contact 2'43 is made adjustable in position by the screw 2%"and is connected to control circuit 233 through a-manu'ally opera---ble switch 2'45. -By setting contact-2 i3'to engage contact'242 at aparticularupper tolerance limit of cabintempera'ture an-d with switch265 closed, cabin diiierential pressure may be limited to a value whichwill prevent seriouscabin temperature-discomfort in favor of cabinpressure comfort, even though unreasonable schedules of .oabin pressurecontrol are attempted during extremely warm weather. A reduction ofcabin temperatureis normally associated with a reduction of cabinpressure since incoming air is heated by the supercharger somewhat inproportion to the cabin diiierentialpressure. For most high altitudeoperations in normal or cold weather additionalheat'willbe required tomaintain a Fqtemperature level even vwhile flying with fullcabinpressurization. This additional heat 'willbe supplied 'byusome formof heater, not shown, "but associated with theair temperatureconditioner 28 shown in Figurel.

.For extreme warm weather operations with no cooling facilities for theincoming air, cabin temperature may become excessive upon attemptingextreme pressurization and then cabin overheat thermostat 59 may be madeoperative to automatically try to reduce cabin pressure to lower thecabin temperature below the set limit if ever this set limit .isexceeded. With intercoolers and an expansion turbine refrigerationapparatus installed in'air temperaturecon- -ditioner 28 to cool thecabin to comfortable temperature during extremely :warm weather, cabinoverh-eatthermostat 59 automatically tries tolower thecabin pressure-and,jas a consequence, make available more of the supercharger powerfor refrigeration if ever'the cabin temperature exceeds the setupperlimit. The-overheatther- 15 mostat may, for example, be set to 85or 90 Fahrenheit.

Landing gear switch 60 is mounted on the landing gear of the aircraftand is electrically grounded as shown in Figure 1. The landing gearshown is of the conventional retractable design in current use on mostall present day commercial aircraft. A hydraulic pneumatic shock strut246 is movable in a cylinder housing 24! in such manner that the strutand a wheel 248 attached to this strut move upward under load as uponapplication of the aircraft weight against the ground in landing. Inlanding position the strut and wheel move downwardly with respect to theaircraft when the aircraft weight is lifted off the gear as occurs whenthe aircraft becomes airborne. The strut 246 is provided with a contact249 which is electrically grounded. Cylinder housing 24! carries acontact .255 which is so spaced from contact 249 as to engage it onlywhen weight of the aircraft is on the landing gear. A normally closedmanually operated switch 252 permits the landing gear switch to beremoved from the system if desired, as when testing the sytem with theaircraft on the ground.

Inadvertent settings of regulators 56 or to cabin altitudes below thoseof the level of the landing field are thus prevented from either causingcabin pressurization prior to flight, even though the system isotherwise made operative, or from maintaining cabin pressure once theaircraft lands.

Cabin pressure apertures I2, I52 and 256a of regulators 56, 51 and 58respectively, are communicated to cabin pressure through an anticipatorsystem in order to render these control elements extremely sensitive totransient changes in pressure during cabin pressure operation. By thissystem trends of cabin pressure and potential surges in cabin pressureare made reactive upon the regulators prior to any appreciable change inthe cabin pressure. The anticipator system per se forms no part of thepresent invention and is disclosed and claimed in United States PatentsNos. 2,407,257 and 2,407,258 issued from my copending applications,Serial Numbers 429,901 and 446,039. For this reason, only those detailsof the anticipator system as are necessary to a full understanding ofthe present invention will be described herein.

This system, referring now to Figure 1, comprises a low capacity airflow circuit arranged in parallel with the cabin ventilation circuit. Inthe anticipator circuit air enters the Pitot tube 253 pointed upstreamin duct 22, passes through a conduit 254, and discharges into a venturi255 located in the upper duct 29. The pressure in Pitot tube 253 isslightly higher than cabin pressure by the amount of ram and pressuredrop from this position in duct 22 to the cabin. The pressure in venturi255 is slightly less than cabin pressure by the amount of Venturisuction and pressure drop to this position in duct 29 from the cabin. Asmall casing 256 is interconnected into the conduit 254 at asubstantially mid-point therein. The pressure within the casing 256 issubstantially equivalent to cabin pressure whenever stabilized andequivalent air flows are passing into and out of the cabin. Conduits25'! and 258 leading from regulators 56 and 58 respectively, areconnected into the casing 256 so that the interiors of the instrumentsare subject to pressure variations in the anticipator system. At a pointin conduit 254 adjacent the venturi 255 wherein the pres- 16 sure isless than cabin pressure and very nearly equal to the pressure venturi255, a conduit 259 leading to the pressure responsive element ofregulator 5'! is connected into the conduit 254.

The cases of either or both the regulators 56 and 58 as well as thepressure responsive element of regulator 51 may be made directlyresponsive to the cabin pressure rather than to the pressure in theanticipator system, if at any time it is so desired, by suitableoperation of plug valves 26!, 262 and 263, respectively.

An adjustable restrictor valve 264 is provided in conduit 254 in orderthat the non-transient or normal static pressure at the medial positionin conduit 254 may be readjusted manually to equal cabin pressure eventhough minor variations are more permanently made in the efiectivepressure drop through duct 22 downstream of the Pitot 253 as may beeffected by ventilation distribution adjustment, or upon any minorchange in the pressure drop in duct 29 upstream of venturi 255 as may beefiected by changes in leakage through the seams of the cabin, Arelatively small structural seam leakage is to be expected and it isfurther to be expected that this small leakage will increase somewhatduring the life of the aircraft.

The regulators 56 and 58 are connected into the anticipator system atthe substantially neutral midway point as hereinbefore brought outbecause these regulators require close control of an absolute cabinpressure and. are preferably made sensitive to that exact pressure atthe same time obtaining equal response to changes in cabin in-fiow andout-flow. However, regulator 51 is preferably connected to theanticipator circuit -at a point closely adjacent to the venturi 255 inorder that an even greater speed of response is attained with respect tochange in cabin outflow as controlled by valve 32 than is attained withrespect to change in the flow of air into the cabin.

As a safety feature for protection of the structure of the airplanecabin in case of emergency, a relief valve 266 is furnished on the cabinwall as shown in Figure 1. The setting of this simple spring-loadedvalve is adjusted by means of adjustment nut 26'! to a value above thatlimited by pressure limits regulator 58. A handle 268 is provided topermit normal operation of the valve to keep it free and to check it forfreedom. A lead screw 269 is mounted adjacent the handle to hold the.valve open if so desired in emergency.

Ratio to flight cabin pressure regulator 56 presents a dial and handsarrangement to the operator as shown in Figure 5. The dial is arrangedfor rotation concentric with the disc I36 carrying the hands I31 andI38. Numbers representing standard pressure altitudes are so marked onthe dial 90 that the difference between the standard pressurescorresponding to any two altitude readings indicated thereon by thehands 13! and I38 is exactly equal to the predetermined limitingdifferential pressure for the airplane cabin at the altitude indicated.Hand I3! is labeled as showing ratio limit flight altitude and hand I38is labeled as showing ratio limit cabin altitude. A fixed indicator 2'is mounted at the top of the face to mark a pressurizing altitudesetting as read on dial 90. Some desired index marks such as shown at212 on indicator 2' and hand I38 can be used to indicate the range ofcabin pressure change which may be expected while flight is variedbetween 1 pressure curve of any flight altitude.

altitudes indicated by the index marks 213 carried by the indicator 2'IIand hand I31.

Time rate of change cabin pressure regulator 51 presents a dial andhands arrangement to the operator as shown in Figure 7. Pressure changerate setting for limiting cabin climb is indicated by needle I85 overthemarkings on dial I83 while pressure change rate setting for limitingcabin descent is indicated by needle I89.

Control of cabin pressure as regulated by my cabin pressure controlsystem herein described is intended to always be limited by regulator 58within a maximum predetermined differential pressure which is the safemaximum for the cabin structure, and within a maximum predeterminedcompression ratio which is the safe and attainable maximum for the cabinpressure air supply superchargers. Typical values for such limits are8.5 inches of mercury differential pressureand 1.75 compression ratio.This differential pressure represents an 8000 foot efiective cabinaltitude at 20,000 feet flight pressure altitude and the 1.75compression ratio represents an effective 11,300 foot altitude at 25,000feet flight pressure altitude.

There is shown in Figure 6 graphic illustrations of pressures which arelimited by regulator 58 as well as the control functions of ratio toflight standard atmosphere as a function of flight altitude is indicatedbythe curve 215 and so labeled. Flight altitude is scaled inthousands offeet along the abscissa and pressure is sealed in inches of mercuryabsolute along the ordinate of the graph. The limit cabin differentialpressure. of. 8.5 p. s. i. with respect to the atmospheric pressure isshown by the curve 216. Note that the vertical or ordinatedistance'between this curve and the atmospheric pressure curve is aconstant fixed quantity for all flight altitudes. The limit cabincompression ratio of 1.75 is shown by the curve 211 and is so labeled.It should be noted that the total ordinate of this curve is a fixedmultiple of the ordinate for the atmospheric Also note that the limitcurves so chosen in this example case intersect at an altitude of 25,000feet. Below 25,000 feet'maximum differential pressure is made the limitcondition but above 25,000 feet the compression ratio becomes thelimitation. To produce this control limitation, post 232 on theexpandable side of aneroid 225 in pressure limits regulator 58 as shownin Figure 10 must, for the above control settings, remain disengagedfrom control arm 2I0 until 25,000 feet is attained and must becomeengaged at all altitudes above 25,000 feet with such force as to reducethe differential pressure acting on differential pressure capsule 201 inaccordance with the pressure indicated by the difference in ordinates ofthe compression ratio limit curve andthe atmospheric pressure curve 215of Figure 6.

Connecting the atmospheric pressure curve 275 and the cabin differentialpressure curve 216 in Figure 6, there is'a horizontal pressure controlcurve A. This curve intersects the atmospheric curve at 2,000 feetaltitude and intersects the cabin differential pressure curve] at 11,600feet altitude. Control curve A represents a schedule of cabin pressureduring transition of flight altitude from 2,000 feet to 11,600 feet andfor this schedule cabin pressure is seen to remain con- Curve A canfurthermore be considered as one cabin pressure regulator 56. Thepressure of the as described heretofore.

component of an overall cabin pressure schedule for aircraft flightsfrom below 2,000 feet to well above 30,000 feet. For such a schedule,cabin pressure would remain equal to atmospheric pressure during flightup to 2,000 feet as represented by the atmospheric pressure curve 215.During flight between 2,000 feet and 11,600 feet, the cabin altitudewould remain substantially constant at 2,000 feet as represented bycurve A. For flight from 11,600 feet to 25,000 feet the cabindifferential pressure would remain constant at 8.5 inches of mercury asrepresented by the differential pressure curve 2'16.

Above 25,000 feet the cabin pressure would be controlled at a fixedratio of 1.75 times the pressure of the ambient atmosphere asrepresented by the compression ratio limit curve 217. This controlschedule can be accomplished with the cabin pressure control systemshown in Figure 1, first by setting the dial 00 of ratio to flight cabinpressureregulator to a pressurizing altitude of 2,000 feet, this settingbeing shown in Figure 5. Next, time rate of change cabin pressureregulator 51 is to be set to reasonable rate comfort limits, forexample, with hand I84 at 600 feet per minute climb and with hand I89 at400 feet per minute descent as shown in Figure 7. With switches GI and42 in closed positions as shown in Figure 1, the above schedule is nowoperative. The bracket I22 in regulator 56 will for this schedule lineup in the horizontal zero position shown for it in Figure 3.. For flightaltitudes below 2,000 feet, aneroid capsule 53 in regulator 56 willremain sufficiently-collapsed to hold the control arm against contact61, thereby closing valve opening circuit 5i which results in controlvalve 32 being openbut stationary as a result of ,the open position oflimit switch 43. .As the pressurizing altitude of 2,000 feet is reached,aneroid capsule 53 has expanded sufficiently to balance control arm 65between contacts 56 and 6?. As further ascent is made, aneroid capsule63, expands, engaging control arm 65 with contact 66, thereby energizingvalve closing circuit 59 to the end that the valve 32 moves toward aclosedposition such that cabin absolute pressure is increasedsufficiently to collapse the aneroid capsule back to a positionof'contact balance. A 2,000 foot cabin altitude is thus maintained. 2

The scheduled pressure limits are maintained above 11,600 feet flightaltitude by regulator 58 If a 600 foot per minute rate of cabin climb isnot exceeded during ascent, andif a 400 foot per minute rate of cabindescent is not exceeded during descent,,regulator 5! will remaininoperative in the system. If these rate limits are exceeded, rate ofpressure change regulator 5'! will, by means of control arm I54, actuatevalve closing circuit 45 in opposition to valve opening circuit 5| orvice versa, thereby opening the control, of motor 36 from relay 45 andstopping all action on valve 32 until cabin pressure a number ofschedules possible with the system of the present invention in additionto that represented by curve A, havev been plotted thereon.

Control of cabin pressure in accordance with a schedule along any ofsloping curves such ascontrol curves B C D E F can be represented by afractional ratio. An ordinary ratio to flight control would berepresented by the following ratio where P is the flight absolutepressure at the pressure altitude at which pressurization of the cabinis to begin, and k1 is a predetermined constant which for example mayhave a useful range from to .6. In the latter case this would beequivalent to saying that the descrease in cabin pressure above apredetermined pressurizing altitude will be .6 times the decrease inflight altitude pressure above this same predetermined point. Ifregulator 56 were to consist of two coacting aneroid units, onesensitive to cabin pressure and one sensitive to flight pressure, thenthe above expression for the ratio control would be clearly applicable.However, since regulator 56 comprises an aneroid sensitive to cabinpressure and a differential pressure capsule exposed to the differencebetween cabin pressure and flight pressure, the above expression may bemore clearly applied in the form:

(P) (cabin absolute pressure) (cabin absolute pressure) (flight absolutepressure) where P is the flight absolute pressure at the pressurealtitude at which pressurization of the cabin is to begin, and k2 is apredetermined constant which for example may have a useful range from 0to 1.5. The expression for 701 and 762 define identical schedules ofcabin pressure in that they each represent a straight line when plottedon a graph of cabin pressure as one ordinate and flight pressure as theother ordinate. In Figure 6, the form of cabin pressure control in whichthe change of cabin pressure is a direct ratio to the change of flightpressure is represented by the sloping curved dotted lines. The slopingcurves, although straight when plotted on a graph of cabin pressure asone ordinate and flight pressure 7 as the other ordinate are notstraight when plotted as in Figure 6 but are bent with a greater slopeat lower altitudes than at higher altitudes due to the form of thecoordinate plot of the abscissa and ordinate in Figure 6. The solidstraight curves in Figure 6 however represent ratio to flight pressurecontrol in which the change of cabin pressure is a direct ratio to thechange in flight altitude and not flight pressure. This control is aconsiderable improvement over the form of the control represented bysloping curves since during the operation of aircraft climbs anddescents are normally gauged by instruments which read in terms ofaltitude. Thus, for the greatest cabin comfort to passengers, the changeof cabin pressure should be controlled at a minimum rate in relation tothe pressure altitude variation of the aircraft and not its flightpressure variation. This type of control can be best defined by itsstraight line relationship in Figure 6 but can also be expressed:

where P is the flight absolute pressure at the pressure altitude atwhich pressurization of the cabin is to begin, and k3 is a predeterminedconstant. If pressures are expressed in inches of mercury and altitudesare expressed in thousands of feet the useful range of this ratio 703 isfrom 0' to about .00007.

The equations for 701 and k2 represent a ratio to flight pressurecontrol which can be defined as a straight line on a graphic plot ofcabin pressure as a function of flight pressure. The equation for itsrepresents a ratio to flight pressure control which can be defined as astraight line on a graphic plot of cabin pressure as a function offlight altitudes. Two other forms of ratio control are similar to thetwo previously mentioned but as should now be understood are far lessdesirable. These are, first, one which plots a straight line for cabinaltitude varying as a function of flight altitude, and secondly, onewhich plots a straight line for cabin altitude varying as a function offlight pressure.

The straight curves are then preferred control schedules to that shownby the sloping curves. However, since the fractional expression for theratio 701 in the sloping curves is simple to express, this general rationotation will be used for the ratio to altitude curves. For example, asshown by the table in Figure 6, the simplified approximate value of theratio for curve B is A; or .25. Its exact value would have to beexpressed by k3.

Control curve B represents a schedule of cabin pressure duringtransition of flight altitude from 2,000 feet to 15,500 feet and forthis schedule, cabin pressure is seen to vary from a value of 27.82inches of mercury absolute (2,000 feet altitude) to a value of 24.98inches of mercury absolute (4,900 feet altitude) during flight from2,000 feet to 15,500 feet.

This cabin pressure control schedule can be accomplished with the cabinpressure control system shown in Figure 1, first by setting the dial 9!!of ratio to flight cabin pressure regulator 56 to 2,000 feet. Next theratio limit flight altitude hand I3! is to be set to 15,500 feet. Theratio limit cabin altitude hand 138 will then automaticaly read 4,900feet. The time rate of change cabin pressure regulator 57 may be set asin the previous example to limits similar to those shown in Figure 7.This control schedule including curve B is now operative.

Other typical ratio control schedules possible with the system of thepresent invention are indicated by control curves C, D, E, and F inFigure 6. The curves show the desired straight line form of control asplotted thereon. For curves A and C the pressurizing altitudes are 2,000feet, the ratio limit flight altitudes are 11,600 and 25,200 feet,respectively, and the net or average control ratios are 0 andrespectively. For curves D, E and F the pressurizing altitude is 6,000feet, the ratio limit flight altitudes are 17,300, 22,000, and 28,500,respectively, and the net or average control ratios are again 0, andrespectively. The approximate average ratios are marked around the outerrim of the face of regulator 56 as shown in Figure 5. An arrow 219 onthe disc I36 indicates the average ratio reading as determined byadjustment of the hand 131 of the regulator.

For control along curves A or D of Figure 6, the position of theU-shaped bracket I22 and regulator 56 is moved through adjustment of theknob 16 to a position substantially parallel to control link H3 as bestseen in Figure 3. For

control along curves B or E an intermediate angular position of thebracket I22 approximately as shown in Figure 4 is to be used. Forcontrol along curves O or F a larger angle of this bracket with respectto control link H3 is to be used.

Action of aneroid assembly 63 and differentialpressure capsule assembly64 is directly related to pressure changes, not altitude functions, sothat when regulator 56 is set for ratio control, the ratio scheduleswould normally be in accordance with the ratio curves shown in brokenlines and identified in Figure 6 as B C E and F1. The slope of each ofthe curves B1, C1, E1 and F1 is greater than the slope of the straightline curves B, C, E and F in the low altitude range of each curve wherecabin differential pressure is low, and the slope is greater in thehigher altitude range where higher cabin differential pressures exist.Regulator 56, however, includes means for so adjusting the control armlinkage from the differential pressure capsule that action of the ratioproducing or slope producing elements is decreased at low differentialpressures, and is increased at higher differential pressures. Thus theratio control curves shown in broken lines are in effect bent to thestraight line slope of the curves B, C, E and F.

This adjustment is made by moving differential pressure capsule assembly64 to the right as viewed in Figure 4 along the Slide 93 to a positionin which link II? subtends a substantial angle from the vertical. If thelink H1 is moved to a position in which the angle is within the range of35 to 50 and the length of the link II? is relatively short, thenecesary geometric arrangement is had to modify the ratio control curvesto the solid curves desired. It will be seen that differential pressurecapsule assembly 64 contracts substantially vertically under theinfluence of increasing differential pressure. During its contraction inthe low differential pressure range, it will cause pin H5 to rotateabout pins I2! at a relatively slow rate due to the large angle betweenlink In and the direction of capsule movement. However, as higherdifferential pressure is reached, link Ill moves toward the vertical orin the path of capsule motion and a greater rate of rotation of pin I I5about pins I2I is obtained thereby increasing the slope of the ratiocontrol curve as desired.

Thus, for example, for the schedule represented by the curve B, as 2,000feet ambient altitude is reached, aneroid capsule 63 will have Texpanded sufficiently to balance control arm 65 between contacts 66 and61. As further ascent is made, aneroid capsule 63 expands and movescontrol arm 65 into engagement with contact 65 to close circuit 49,which as should now be understood results in closing movement of valve32 to a position such that a difiierential of cabin pressure aboveambient pressure is built up, and since cabin absolute pressure isthereby increased, the aneroid 63 starts to collapse back to anequilibrium position. Now, however, the contraction of capsule 64, underthe influence of the dilferential pressure acting thereon, rotates theyoke 1 I6 counter-clockwise about pins l2l, pulls control arm supportand control link II3 to the right and moves control arm 55 towardcontact 67. As the support bracket I22 in this schedule will have beenadjusted to a position approximately as shown in Figure 4, the combinedaction of aneroid capsule 63 and differential capsule 64 on control arm65 is such as to balance it between contacts thereby satisfying thecontrol curve B.

It may be noted in Figure 6 that control curve F connects theatmospheric pressure curve to the compression ratio limit curve withoutlimitation or intersection by the differential pressure limit curve.This simply means that the compression ratio limit is reached beforereaching the maximum differential pressure limit. The ratio limit flightaltitude reading furnished by the sively decreases. Operation of thecabin pressure control sys- 22... hand I31 of the regulator 56 stillindicates the correct limit reading, however, since the peripheralaltitude markings on dial 530 are spaced so that a fixed anglerepresents a fixed pressure difference in the range from 0 to thecompression ratio limit intersection (which is 25,000 feet in theexample shown) and are spaced so that a fixed angle from any altitudemarking below this intersection altitude to any marking above thisintersection altitude represents a fixed compression ratio. On the dial90 the space between altitude markings is thus shown to progressivelydecrease from 0 to 25,000 feet, after which the unit spacing ismagnified and again progrestem shown in Figure 1 has been explained forcontrol with ratio to flight schedules. For these schedules, time rateof pressure change regulator 51 was used only as a veto device toprevent the occurrence of uncomfortable rates of pressure change duringresetting of ratio to flight regulator 56 on the link. Now another typeof control can be accomplished with this control system, namely, timerate of pressure change con trol in which operation regulator 51 becomesthe primary controller, and regulator 56 merely stops the controllercabin climb on descent at a set altitude. Pressure limits regulator 58always maintains control over the extreme limit to 22,000 feet at 500feet per minute, maintain ing level flight for 68 minutes, and finallydescending to airport Z at 4,000 feet at the rate of l,000 feet perminute. The lower limit of cabin altitude dictated during this flight bythe example value of 8.5 inches of mercury peak cabin diferentialpressure is shown by the curve JKLMNOPQRS. Now by setting both hands I84and I89 of regulator 51 to the 500 feet per minute climb mark, a cabinclimb along a curve OT may be accomplished. To prevent the cabin fromcontinuing to climb right on up to the flight altitude it may be stoppedat the 9,000 foot level indicated by curve TU by setting regulator 56 tozero ratio and to 9,000 feet pressurizing altitude. The connection ofvalve opening circuit 59 to regu ator 56 is automatically broken by theseparation'of contacts I62 and I90 in regulator 51 when both indicatorsare set to a climb value.-

ule would be in accordance with curve OB KC the set rate of climb beinginterrupted from B to K by the action of pressure limits regulator 58. e

Curve TU once obtained in the cabin pressure control schedule may bemaintained by the setting above or may be maintained by two alternateprocedures, the first of which is to set the time rate of pressurechange hands to zero, or secondly to set hand I84 anywhere in the climbrange and hand I89 anywhere in the descent range thereby allowingregulator 56 to maintain the 9,000 feet cabin altitude. At point U onthis curve, pressure limits regulator 58 would take over continuingthrough UOPV.

A time rate of pressure change control of descent may be started at apoint W or X, for example, on curve VX. If both hands of regulator 51are set to 250 feet per minute descent at point W and if regulator 56 isset to an equalizing altitude of 4,000 feet or lower, the cabin willdescend in accordance with curve WA If both hands of regulator 51 areset to 400 feet per minute descent at point X, and if regulator 56 isset to a pressurizing altitude of 4,000 feet and to zero ratio, then thecabin will descend in accordance with curves KY and YZ. The altitudelimit action of regulator 56 is accomplished as a result of automaticseparation of contacts l6! and 194 in regulator 57 at the time that handI84 is set into the descent range, so that only the valve openingcircuit 5| is active during the descent.

Operation of the control system in Figure l on a time rate of pressurechange schedule is generally less desirable than operation on a ratio toflight schedule. The reasons are obvious for, as pointed outhereinbefore, several different settings of time rate of pressure changeregulator must be made at specific times during the flight. This isoften inconvenient and a burden to the flight engineer or otheroperator. The continuous setting and resetting at times may causeunnecessary discomfort to the passengers and the failure to remember tostart the cabin down at the correct time unnecessarily complicates thechange of cabin pressure. For example, in Figure 9, descent of the cabincannot be started much before point W or the differential pressure limitwill interrupt it. If the start of cabin descent is delayed 6 or 8minutes, then a much more abrupt descent must be made or else the cabinwill become equal ized with flight pressure during the descent and ahigher rate of descent will automatically be imposed.

It is furthermore difflcult to ascertain the exact time that theairplanes flight will reach a predetermined altitude. On the other hand,with ratio to flight control the regulator settings may very often neverhave to be changed throughout the flight and in cases where airports ofdeparture and destination differ greatly in altitude, regulator settingscan be changed to meet the most comfortable schedule any time prior tothe start of descent. Descent of the cabin in ratio to flight controlautomatically starts when the airplane begins to descend. However, thereare times when a fast descent of limited extent may be necessary throughan opening in the overcast. For this and other similar occasionalconditions it is important and desirable that the time rate of pressurechange control be readily available with convenient means of making itoperative over the ratio to flight type of control.

Although the now preferred embodiment of the invention has been shownand described herein, it is to be understood that the invention is notto be limited thereto for the invention is sus ceptible to changes inform and detail within the scope of the appended claims.

I claim:

1. An aircraft compartment pressure control comprising: means fordelivering air to said compartment under a pressure greater than ambientflight pressure; means for discharging air from said compartment; meansfor varying the rate of air discharge from said compartment relative tothe rate of air delivery to said compartment whereby the absolutepressure within said compartment may be varied; means, including a firstcapsule subject to cabin absolute pressure and a second capsule subjectto cabin differential pressure coacting through an interconnectinglinkage system, made operative upon said aircraft reaching apredetermined pressure altitude for controlling said last-named meansfor regulating the absolute pressure in aid compartment in such a manneras to change said pressure in inverse straight-line proportion tochanges in pressure altitude of said aircraft above said preselectedpressure altitude; and means for preselecting the pressure altitude atwhich said controlling means is made operative; said preselecting meansbeing changeable during flight of said aircraft and operableindependently of said control means whereby said pressure altitude atwhich said control means is made operative may be altered during flightof said aircraft without altering the said proportion.

2. An aircraft compartment pressure control comprising: means fordelivering air to said compartment under a pressure greater than ambientflight pressure; means for discharging air from said compartment; meansfor varying the rate of air discharge from said compartment relative tothe rate of air delivery to said compartment whereby the absolutepressure within said compartment may be varied; control means, includinga first capsule subject to cabin absolute pressure and a second capsulesubject to cabin differential pressure coacting through aninterconnecting linkagesystem,made operative by said aircraft reaching apreselected pressure altitude for operating said last-named means tomaintain the absolute pressure within said compartment at a value abovethe ambient pressure at all practical flight altitudes Within theoperating limits of said control, said control means regulating theabsolute pressure in such a manner as to change said pressure in inversestraight-line proportion to changes in the pressure altitude of saidaircraft above the preselected pressure altitude; means for preselectingthe pressure altitude at which said control means is made operative; andmeans for altering the proportion, said altering means being operativeindependently of said preselecting means whereby said proportion may bealtered during flight of said aircraft without altering the preselectedpressure altitude at which said regulating means is made operative.

3. An aircraft compartment pressure control comprising: means fordelivering air to said compartment under a pressure greater than ambientflight pressure; means for discharging air from said compartment; meansfor varying the rate of air discharge from said compartment relative tothe rate of air delivery to said compartment whereby the absolutepressure within said cabin may be varied; means, including a firstcapsule subject to cabin absolute pressure and a second capsule subjectto cabin differential pressure 00- acting through an interconnectinglinkage system, for controlling said last-named means in such manner asto change the absolute pressure in said compartment in inversestraight-line proportion to changes in flight pressure altitude; meansmade operative by a preselected rate of cabin absolute pressure changefor controlling said varying means for inhibiting the time rate ofchange of said cabin absolute pressure during changes of altitude ofsaid aircraft which, through operation of said first control means,produce a rate of cabin pressure change in excess of said preselectedrate of change; means for preselecting the rate of change of said cabinpressure at which said last-named controlling means is made operative;and means made operative upon the attainment of a predetermineddiiTerential of cabin absolute pressure above externalabsolute pressurefixed by the structure of the aircraft to override both of saidcontrolling means and operate said varying means to maintain suchdifferential as long as external absolute pressure is such as to tend toincrease the differential.

4. An aircraft compartment pressure control iii comprising: means fordelivering air to said .compartment under a pressure greater thanambient flight pressure; means for discharging air from saidcompartment; means for varying the rate of air discharge from saidcompartment relative to the rate of air delivery to said compartmentwhereby the absolute pressure within said cabin may be varied; means,including a first cap-' sule subject to cabin absolute pressure and asecond capsule subject to cabin differential pressure coacting throughan interconnecting linkage system, for controlling said last-named meansin such manner as to change the absolute pressure in said compartment ininverse straight-line proportion to changes in flight pressure altitude;means for controlling said varying means for inhibiting the time rate ofchange of said cabin absolute pressure during changes of altitude ofsaid aircraft which produce a rate of cabin pressure change in excess ofsaid preselected rate of change; means for preselecting the rate ofchange of said cabin pressure at which said lastnamed controlling meansis made operative; means made operable by the attainment of apredetermined differential of cabin absolute pressure above externalabsolute pressure for over riding both of said aforesaid controllingmeans and thereafter operating said varying means to maintain saidpredetermined differential substantially constant as long as externalabsolute pressure is below a predetermined minimum value; and meansoperable in accordance with a selected ratio between cabin absolutepressure and external absolute pressure, also operatively connected tosaid varying means, to control cabin absolute pressure so as to maintainsaid ratio substantially constant so that said air delivery means isoperated within its capabilities throughout even the highest altituderange of the aircraft.

5. An aircraft compartment pressure control comprising: means fordelivering air to said compartment under a pressure greater than ambientflight pressure; means for discharging air from said compartment; meansfor varying the rate of air discharge from said compartment relative tothe rate of air delivery to said compartment whereby the absolutepressure within said compartment may be varied; control means, including a first capsule subject to cabin absolute pressure and a secondcapsule subject to cabin differential pressure coasting through aninterconnecting linkage system, for operating said last-named means tomaintain the absolute pressure within said compartment as astraight-line function of flight pressure altitude; atime-rate-of-pressurechange means operable at a preselectedrate ofchange to preclude the normal action of; said control means whenever thechange in pressure alticess of the preselected rate; means forpreselecting said rate of change at which said means is operable; andmeans carired by said time-ratebf-pressure-change means for renderingsaid control means inoperable and for rendering saidtime-rate-of-pressure-change means operable to supersede the action ofsaid control means, whereby said time-rate of pressure change meansoperates said varying means to change the absolute pressure within saidcompartment at the preselected rate of change.

6. An aircraft compartment pressure control -comprising: means fordelivering air to said compartment under a pressure greater than ambientflight pressure; means for discharging air from said compartment;means'for varying the rate of air discharge from said compartmentrelative to; the rateof air delivery to said compartment whereby theabsolute pressure within said cabin may be varied; means, including afirst capsule subject to cabin absolute pressure and a second capsulesubject to cabin differential pressure coacting through aninterconnecting linkage system, for controllingsaid varying means insuch a manner as to change the absolute pressure in said compartment ininverse straight-line propor tion to changes in pressure altitude of theaircraft; -normally inoperative rate of pressure change means, madeoperative by the aircraft ascending or descending at a rate such as tocause operation of said primary control which pro duces changes in theabsolute pressure of the compartment in excess of preselected rates ofchange of cabin pressure, for controlling said varying means to maintainthe rate of pressure change within the cabin to said preselected rate ofchange; and means for preselecting said rates of change of cabinpressure.

'7. An aircraft compartment pressure control comprising: means fordelivering air to said compartment under a pressure greater than ambientflight pressure; means for discharging air from said compartment; meansfor varying the rate of air discharge from said compartment relative tothe rate of air delivery to said compartment whereby the absolutepressure within said cabin may be varied; means, including a firstcapsule subject to cabin absolute pressure and a second capsule subjectto cabin differential pressure coacting through an interconnectinglinkage system. for controlling said varying means in such a manner asto change the absolute pressure in said compartment in direct proportionto changessure change means, made operative by the air-' craft ascendingor descending at a rate such as to cause operation of said controllingmeans which produces changes in the absolute pressure of the compartmentin excess of preselected rates of change of cabin pressure, forcontrolling said varying means to maintain the rate of pressure changewithin the cabin to said preselected rate of pressure change; and meansfor manually adjust ing said rate of'pressure change means to cause saidrate of pressure change means to control the rate of cabin pressurechange independently of the rate of change of the pressure altitude ofthe aircraft.

8. An aircraft compartment pressure control comprising: airflow meansfor circulating the flow of air under pressure through the compartmentand including pressurizing inlet means, outlet means, and valve meansfor controlling the discharge of air through said outlet means; meansfor regulating said valve means so as to control the absolute pressurewithin said compartment as a straight line function of pressure altitudeof said aircraft; and anticipating control means sensitive to transientchanges in diiferential between quantities of flow in said inlet andoutlet means, respectively, adapted to produce in said regulating meansa response to said transient changes before any substantial change inthe absolute pressure of the cabin can occur as a result of saidtransient changes.

9. An aircraft compartment pressure control comprising: airflow meansfor circulating a flow of air under pressure through the compartment,said airflow means including inlet, outlet, and valve means forcontrolling said flow; means responsive to both cabin absolute pressureand flight pressure altitude so organized and arranged that the cabinabsolute pressure is controlled to change at a predetermined ratio tothe variation of pressure altitude of the aircraft from a preselectedpressure altitude through regulation of said valve means, said valvecontrolling means including means defining a pressure chamber ofrestricted volume; and anticipating control means adapted to transmit tosaid chamber a pressure derived as a differential result of the flows insaid inlet and outlet means, respectively, whereby to normally maintainin said chamber a pressure equal to cabin pressure and varying in stepwith cabin pressure in response to gradual changes in said inlet andoutlet flows but effective in re sponse to transient differences in saidflows to change the pressure in said chamber in anticipation ofcorresponding changes in the pressure of the air within the compartmentand to thereby actuate said valve controlling means to effect acorrective adjustment of said valve means so as to anticipate andprevent said corresponding changes in pressure in cabin air.

10. An aircraft compartment pressure control comprising: means fordelivering air to said compartment under a pressure greater than ambientflight pressure; means for discharging air from said compartment; meansfor varying the rate of air discharge from said compartment relative tothe rate of air delivery to said compartment whereby the absolutepressure within said cabin may be varied; means, including a firstcapsule subject to cabin absolute pressure and a second capsule subjectto cabin differential pressure coacting through an interconnectinglinkage system, made operative by said aircraft reaching a preselectedpressure altitude for controlling said last-named means, therebyregulating the absolute pressure in said compartment to change saidpressure in inverse straight-line proportion to changes in pressurealtitude of said aircraft above the preselected pressure altitude; meansfor preselecting said pressure altitude; means including manuallyoperable means for changing said proportion, said changing means beingoperable independently of said preselecting means whereby saidproportion may be varied during flight of saidaircraft without alteringthe said preselected pressure altitude at which said regulating means ismade operative; and means controlled by operation of said manuallyoperable means for indicating at any of the selected proportions theabsolute pressure Within said cabin as simulated cabin pressure altitudeat any particular pressure altitude of said aircraft.

11. In an aircraft: a cabin adapted to be super charged to maintainpressure in excess of that of the ambient atmosphere; means forsupplying air to said cabin; means including a valve for discharging airfrom said cabin; means for controlling said valve to normally controlcabin absolute pressure to change as a direct ratio of changes in thepressure altitude of the aircraft; time-rate-of-pressure-change meansfor supervising and vetoing control action instituted by the first saidmeans; control means for overriding the action of both theaforementioned means and limiting the maximum cabin differentialpressure to predetermined safe limits; means constituting an air flowconduit between said air supplying means and said air discharge meansincluding a portion wherein pressure is higher than cabin pressure andanother portion wherein pressure is lower than cabin pressure; meansrespectively flow-connecting a point in said conduit wherein thepressure is normally equal to cabin pressure to said ratio-to-flightcabin pressure control means and to said overriding pressure controlmeans; and means flow-connecting said timerate-of-pressure-changecontrol means to said conduit adjacent said lower pressure point,whereby to make said pressure control means highly sensitive to air flowsurges and maintain smooth automatic regulation of cabin pressureindependent of transient changes in the air flow in said delivery anddischarge means 12. In an aircraft having a cabin adapted to bepressurized: a pressure discharge valve in said cabin; means foroperating said valve; a cabin: pressure regulating means for controllingsaid valve operating means; an electroresponsive means operable whenenergized to actuate said sponsive means is energized when the gearcomponent is engaging the ground and the valve opened to obviatepressurization of the cabin due to inadvertent setting of said regulatorbelow the pressure altitude of the landing field from causing cabinpressurization prior to flight and for equalizing cabin absolutepressure with ambient absolute pressure upon landing of the aircraft;and means including a time-rate-ofpressure-change regulating means forlimiting the rate of change of cabin absolute pressure upon landing ofthe ing a predetermined temperature to render said regulating meansinoperative and to operate said valve actuating means to open saidvalve; and means for supervising and limiting the valve action to limitthe rate of pressure decrease in the compartment.

